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The target of this paper is to design a novel water-vapor sensor used in humidity transducer system based on high-quality
factor (Q) spherical microcavities, which at present obtained the highest Q value induced by surface tension. Sensitive
mechanics and its high sensitivity are discussed according to optical loss sensitization to coupling system composed of a
microsphere cavity and a tapered fiber. After that, fabrication methods of master parts of the sensor in our lab are
introduced. Additionally, a sensing structure composed of a gas-chamber and a vacuum-chamber is also designed.
According to water absorption band, telecom band around 1550nm is used to pump the microsphere optical mode. In a
vacuum environment, a transmitted spectrum is obtained through output end, revealing some information about the core
system including the size of the microcavity and the optical loss of the regime. However, when the core system is placed
in a water vapor environment, the transmitted spectrum will change due to extra optical loss induced by water molecules
absorption, behaving as spectral shift, free spectrum range (FSR) or line width broadening. Contrasting and analyzing
two different spectrums in the two situations, the gas concentration can be deduced. Indispensable experiments were also
carried out, showing that, the two spectrums are different from resonance hump, formant strength and formant line
width. Even an ultra low water vapor concentration induced a measurable output signal, ensuring a high detecting
sensitivity. Certainly, the analyted vapor should not only be water vapor. This kind of sensitive mechanics is versatile.
Changing the pumping light corresponding to the analyted absorption band, we are able to detect a series of vapor.
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